chore: add test optional dependency

pyproject.toml

@@ -34,5 +34,10 @@ dependencies = [
     "PIMS >= 0.5.0"
 ]
 
+[project.optional-dependencies]
+test = [
+    "pytest"
+]
+
 [tool.setuptools_scm]
 write_to = "src/zolfa/nd2reader/version.py"